1. Field of the Invention
This invention relates to a nuclear fuel assembly designed for use in a puressurized water reactor (PWR), which assembly is fitted with a filter member on a bottom nozzle thereof in order to avoid damage of fuel rods due to foreign matters which intrude into the passageway of a primary cooling water.
2. Description of Related Art
A nuclear fuel assembly for use in a PWR is, as well known, composed of a top nozzle and a bottom nozzle; a plurality of fuel rods, and control-rod guide thimble tubes and an instrumentation tube arranged in a transversely spaced relation to extend vertically between the top and bottom nozzles, thus forming a fuel bundle or cluster, the thimble tubes and the instrumentation tube being interconnected rigidly between the top and bottom nozzles; and grids supporting and inserting the fuel bundle in cells thereof and axially spaced apart from one another.
With such a conventional fuel assembly, a typical bottom nozzle is illustrated in FIG. 6 at 30 and constructed of a quadrangle plate body 31 defined with a plurality of threaded bores 33 for fixedly inserting therein the thimble tubes as a skeleton element of the fuel assembly and a multitude of through-holes 34 for flowing water, and four legs 32 vertically extending from the plate body 31 at its corner parts and supporting the plate body 31 at a predetermined distance spaced apart from a lower core plate 40.
In a PWR including the aforesaid fuel assemblies, a primary cooling water is routed from through-holes (not shown) apertured in the lower core plate 40 into the bottom nozzle 30, past the through-holes 34 and then through the fuel assembly, via the interstices between the grids and fuel rods to reach the top nozzle. The cooling water thus having passed through through-holes of the top nozzle is then flowed through a steam generator to reach the lower core plate 40, whereby a cycle of circulation of flowing water through the PWR is performed.
Here, foreign matters such as metal pieces are mingled within the the cooling water system and often get caught between the the fuel rods after passing through the bottom nozzle in its through-holes. The foreign objects thus caught may be vibrated by reason of the current of the cooling water and are likely to damage the fuel rods.
In order to cope with the foreign objects, heretofore, various attempts to improve the trapping capability of the bottom nozzle or the lowermost grid have beem made with a view toward avoiding the entrance of the foreign objects into the fuel assembly.
The improvement of the bottom nozzle in the trapping capability was carried out by reducing the diameter of the through-holes, apertured in the bottom nozzle, for flowing water, or by fitting the bottom nozzle with a filter member. A problem with these approaches, however, is that they entail an increase in the pressure loss of the fuel assembly, and consequently, there was a limitation in diminishing the hole diameter of the through-holes or the mesh diameter of the filter member. Hence, it was difficult to impede the intrusion of such foreign objects that have a smaller cross-section than the size of a projected shadow of the through-holes over a lower core plate.
On the other hand, the improvement of the lowermost grid was conducted by lowering the position of the grid down to a position coming into contact with the top face of the bottom nozzle, or by attaching dimples for trapping foreign objects within the grid cells. The lowered installation of the lowermost grid is designed for dividing the through-holes in the bottom nozzle by the grid straps thereby to impede the intrusion of further foreign objects of a much smaller cross-section into the fuel assembly.
However, these prior art countermeasures against foreign objects have been proved to have little impeding effect on the aforesaid foreign objects that have a smaller cross-section than a maximum projected shadow dimension of the through-holes. More specifically, when flowing water test with a water containing foreign objects was conducted and the bottom nozzle was observed, the prior art measures were effective for foreign objects with a diameter of at least 4 mm in minimum cross-section, but nearly of no use for linear foreign objects with a diameter of 2 mm and downward, e.g. wires having a smaller cross-section.